1. Field of the Invention
The present invention relates to an improved Extrinsic Fabry-Perot Interferometer (EFPI) sensor. More particularly, the EFPI sensor is configured to be disposed in a borehole penetrating the earth.
2. Description of the Related Art
In exploration and production of hydrocarbons, it is often necessary to drill a borehole into the earth to gain access to the hydrocarbons. Equipment and structures, such as borehole casings for example, are generally disposed into a borehole as part of the exploration and production. Unfortunately, the environment presented deep into the borehole can place extreme demands upon the equipment and structures disposed therein. For example, the equipment and structures can be exposed to high temperatures and pressures that can effect their operation and longevity.
Because optical fibers can withstand the harsh environment downhole, sensors using optical fibers are often selected for downhole applications. One type of sensor using optical fibers is the Extrinsic Fabry-Perot Interferometer (EFPI) sensor. The EFPI sensor can measure pressure or temperature for example by measuring a displacement of one optical fiber in relation to another optical fiber.
A prior art EFPI sensor 10 is illustrated in FIG. 1. The EFPI sensor 10 includes a hollow core tube 11. Disposed within the hollow core tube 11 at one end is a single-mode optical fiber 12. Disposed at the other end of the hollow core fiber 11 is a multimode optical fiber 13. A Fabry-Perot (FP) cavity is formed between the ends of the optical fibers 12 and 13 within the hollow core tube 11. The single mode optical fiber 12 provides input light to the FP cavity and receives light reflections from the FP cavity. The multimode optical fiber 13 acts as a reflector. The hollow core tube 11 is configured to guide the optical fibers 12 and 13 to and from each other while maintaining alignment.
Referring to FIG. 1, the input light enters the single mode optical fiber 12 and is partially reflected by a first glass-to-air interface 14 to produce first reflected output light 15. The input light not reflected by the first glass-to-air interface 14 travels through the FP cavity and is reflected by a second glass-to-air interface 16 to produce second reflected output light 17. The first reflection output light 15 interferes with the second reflection output light 17 to create an interference pattern or interferogram that depends on a difference in the optical path lengths traveled by the reflection output light 15 and 17. The intensity of total output light due to the interference pattern is related to the difference between the two optical paths. By measuring the intensity of the total light output at two different times, the displacement of the single mode optical fiber 12 with respect to the multimode optical fiber 13 can be measured. Hence, a property such as temperature or pressure can be estimated by measuring a change in intensity of the total light output.
In order to maintain proper alignment between the first glass-to-air interface 14 and the second glass-to-air interface 16, the prior art EFPI sensor 10 is made with a close tolerance between the outer diameter of the optical fibers 12 and 13 and the inner diameter of the hollow core tube 11. The tolerance is generally less than three microns. Unfortunately, the close tolerance can create friction, which in turn cause hysteresis in the response curve of the prior art EFPI sensor 10.
Therefore, what are needed are techniques to reduce or eliminate hysteresis in EFPI sensors.